Slice pork across the grain into 1 to 2 inch strips, not more than ⅛ inch thick and set aside. Pound the garlic and coriander root in a mortar to a smooth paste. Combine the fish sauce, sugar and water in a small bowl and stir until the sugar's dissolved. Set aside.

Heat a wok, add oil, swirl to cover. Add the cilantro paste and stir-fry until lightly golden. Add the pork and stir-fry until the pink color disappears. Add the white pepper and stir-fry about 1 minute. Don't let it burn. Add the nam pla mixture and stir over moderate heat until the sauce begins to thicken and any stuck portions are loosened, about 1 minute. Add more water if it gets too dry. There should be about ½ cup of sauce.